The goal is to udnerstand the basic mechanisms of visual sensitivity underlying spatial, contrast, and color discriminations. ONe important strategy in the research program is and has been to rigorously work out the implications for visual sensitivity of various anatomical and physiological mechanisms. These analyses lead to hypotheses and theories that can be tested with psychophysical experiments. Our aim is to work out what aspects of spatial, contrast and color discrimination can be accounted for by the properties of light (quantal fluctuations), the optics of the eye, and the sizes, lattice positions, and spectral sensitivities of the photoreceptors. The theoretical basis of the project is an ideal-discriminator theory (based on the principles of signal detection theory). A computer model of the optics of the eye and the receptor lattice is used to determine the quantum catch in each photoreceptor for the two stimuli in a discrimination task. A mathematically ideal discriminator is then applied to the receptor output. In this way one can determine the physical limits to visual sensitivity imposed by the front-end of the visual system. Psychophysical expeirments measureing acuity, hyperacuity, luminace contrast sensitivity and chromastic contrast sensitivity will then be carried out for comparison with the ideal discriminator's performance. This should allow determination of what aspects of human sensitivity can be explained by properties of the stimuli and the front-end of the visual system up to the receptors. This project should provide a significant step forward in understanding the peripheral visual mechanisms and their contributions to overall visual sensitivity. This is important for understanding normal and abnormal spatial and color vision. The ideal-discriminator theories can also be used to detemrine what information for discrimination is lost by disorders of the optics, receptor lattice, or receptor spectral sensitivities. The project will also make significant contributions toward understanding how to discriminate visual signals in Poisson (quantal) noise. This is a problem of considerable current interest in the area of medical optics--diagnostic imaging devices often produce rather noisy images.